Radiation heat transfer of internal motor components by electro-magnetic waves

ABSTRACT

A motor assembly including a housing with a motor portion having an opening for receiving a motor housing/stator assembly, rotor assembly, front and rear end caps and bearings, an encoder coupled to the motor, and an rear cover for enclosing the opening of the motor portion of the housing, the rear cover at least partially surrounding the encoder. At least one of an interior surface of the rear cover or an exterior surface of the encoder comprise a material having an emissivity greater than 0.9.

BACKGROUND

The present exemplary embodiment relates to radiation heat transfer. Itfinds particular application in conjunction with radiation heat transferof electric motor components, and will be described with particularreference thereto. However, it is to be appreciated that the presentexemplary embodiment is also amenable to other like applications.

Electric motors commonly include certain electronic devices mounted orsupported in a common housing with the rotating elements of the electricmotor. Examples of such electronic devices include switching devices,resistors, encoders, etc. It is well known that such electronic devicesgenerally have a limited operating temperature range and that heatgenerated by the electric motor and/or the electronics themselves canresult in unfavorable operating environments. If the thermal issues arenot addressed, the electronic components may overheat and malfunction,or fail altogether.

In some applications, motor output can be limited by thermal issues. Forexample, a motor with an encoder may experience increased temperatureswhen operating at high RPMs. This can be due to increased heat generatedby the motor, as well as self-heating of the encoder as it spins at highspeeds. Current practice is to reduce torque output of a motor whenoperating at the higher RPMs in order to reduce heat output and therebymaintain the encoder at a suitable operating temperature. Such anapproach is less than ideal since it prevents a motor from being used atfull capacity, or requires the use of a larger motor than wouldotherwise be necessary for a given application.

Other approaches have also been developed for addressing thermal issues.For example, fans have been provided for circulating air around a motorhousing to remove excess heat therefrom. While effective, fans increasecost and require additional space. Another approach has been to provideexternally mounted heat sinks that are designed to transfer heat fromthe motor to an exterior of the motor housing, thus lowering thetemperature within the motor housing. Again, such an approach generallyrequires additional space. Still another approach has been the provisionof liquid cooling systems. The cost of such systems, however, isgenerally very high and designing a system with suitable performance foran electric motor is fairly complicated.

BRIEF DESCRIPTION

In accordance with one aspect, a motor assembly comprises a housingincluding a motor portion having an opening for receiving a motor, amotor received in the housing, an encoder coupled to the motor, and anend cap for enclosing the opening of the motor portion of the housing,the end cap at least partially surrounding the encoder. At least one ofan interior surface of the end cap or an exterior surface of the encodercomprise a material having an emissivity greater than 0.9.

The end cap can be aluminum, steel, or stainless steel, for example, andthe interior surface of the end cap can be anodized black. The interiorsurface of the end cap can be painted black. Both the encoder and theinterior surface of the end cap can be anodized black. Both the encoderand the interior surface of the end cap can be painted or otherwisecolored black. Only the encoder can be painted or otherwise coloredblack. The motor assembly can be a food-grade motor assembly having anexterior of the housing painted white with a paint system meets theUnited States Department of Agriculture Food Safety and InspectionService requirements for incidental, indirect food contact.

In accordance with another aspect, an end cap for a motor assemblycomprises a body having an interior surface and an exterior surface,wherein the exterior surface is painted white, and wherein the interiorsurface has an emissivity greater than 0.9. The interior surface can bepainted black. The body can be aluminum, steel, or stainless steel, andthe interior surface can be anodized black, or the interior and exterior(non-food grade) surfaces of the body can be anodized black, and/or theexterior anodized black surface can be painted white. The exteriorsurfaces can be dyed or tinted white.

In accordance with another aspect, a method of increasing heat transferin a motor assembly comprises providing a motor component, providing amotor housing for receiving the motor component, and increasing theemissivity of at least one of the motor component or an interior surfaceof the motor housing by applying a coating, wherein the emissivity isincreased to at least 0.9. The increasing the emissivity can includepainting with black paint, and/or black anodizing, or blackening theinterior surfaces in any manner such as dyeing or tinting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an exemplary electric motor assembly inaccordance with the present disclosure;

FIG. 2 is an exploded view of the motor assembly of FIG. 1;

FIG. 3 is a partially cross-sectional view of the motor assembly of FIG.1;

FIG. 4 is a perspective view of an end cap and encoder with treatedsurfaces in accordance with the present disclosure; and

FIG. 5 is a graph representing torque vs speed for a test motor withvarious configurations represented.

DETAILED DESCRIPTION

Motors used in certain applications, such as food processing (e.g., foodgrade motors), typically have an aluminum housing that is typicallypainted white on the outside per government regulations. In contrast,motors used outside of such applications are generally black. It hasbeen found that, for a given motor, painting the outside of its housingwhite results in less radiation heat transfer from the housing exteriorsurface to ambient. This increases interior motor component temperaturesand results in a significant decrease in motor performance.

In accordance with the present disclosure, interior surfaces of a motorare treated to increase radiational heat transfer from interior motorcomponents to the exterior of the motor. In one exemplary embodiment,the interior surfaces of a motor housing and the encoder itself arepainted and/or anodized black to increase the emissivity of thecomponents and thereby increase radiational heat transfer from theencoder to the exterior of the motor.

With reference to FIGS. 1-3, an exemplary electric motor assembly inaccordance with the present disclosure is illustrated and identifiedgenerally by reference numeral 10. The electric motor assembly generallycomprises a housing 12 in which the internal motor components, includinga rotor 14 and an encoder 16, are supported. The housing is comprised oftwo main components, a motor housing portion 18 and an encoder housingportion, referred to herein as an end cap or a rear cover 20. The motorhousing portion 18 includes a motor compartment or cavity 24 in whichthe rotor 14 (and stator) is supported, and the rear cover 20 defines anencoder compartment or cavity 26 in which the encoder 16 is contained.The interior of the motor housing 12 and encoder compartment 26 areexemplary in nature, and other configurations are possible. For example,a single housing component can include both the motor housing and theencoder compartment.

As best seen in FIG. 3, the encoder 16 generally includes a cylindricalbody 30 that houses electrical components. The encoder 16 is mountedaxially coextensive with a portion of a shaft 30 of the motor rotor 14,and can be configured to sense both shaft RPM and angular position, asconventional. It will be appreciated that the encoder 16 is shown inschematic form in the drawings, and that the specific details of theencoder 16 is not germane to the present discussion.

In a typical motor assembly, both the encoder body 34 and the rear cover20 that surrounds the encoder 16 are metal. In many applications, theencoder body 34 and end cap 20 are aluminum. The rear cover 20 can alsobe steel or stainless steel along with the motor housing. Bare aluminum,however, has poor radiation properties that limit the amount ofradiational heat transfer from the interior of the motor to the ambientenvironment outside the motor housing.

With reference to FIG. 4, and in accordance with the present disclosure,the interior surfaces of the motor 10 including the interior surface S1of the end cap 20 and the outer surface S2 of the encoder body 34 aretreated to increase emissivity and thereby increase the rate of heattransferred by radiation from the encoder to the ambient environmentoutside the housing. It will be appreciated that the term treated isintended to include, but is not limited to, painting, coating, plating,dyeing, tinting, or otherwise changing the surfaces to increaseemissivity over a base emissivity of the bare, untreated material. Forexample, in one embodiment, the encoder body 34 is anodized black andthe interior surface of the rear cover 20 is painted black. In anotherembodiment, both components can be anodized or painted, or both. Instill another embodiment, the end cap is anodized black, or just theencoder exterior is black)As compared to bare aluminum with anemissivity of approximately 0.09, aluminum treated with a black coloringhas an emissivity of approximately 0.9 to 0.95.

In the illustrated embodiment, the encoder and end cap are eachcylindrical. Radiation exchange between cylindrical bodies isrepresented by the equation:

$q = \frac{\sigma \; {A_{1}( {T_{1}^{4} - T_{2}^{4}} )}}{\frac{1}{ɛ_{1}} + {\frac{A_{1}}{A_{2}}( {\frac{1}{ɛ_{2}} - 1} )}}$

Where σ=Stefan-Boltzmann=constant, and for a given geometry, A1 and A2are constant.

For a given Temperature Delta (between Encoder and Cover), the RadiationHeat Transfer, q, simplifies to a function of Emissivities:

${qoc}\; \frac{constant}{\frac{1}{ɛ_{1}} + {{constant} \cdot ( {\frac{1}{ɛ_{2}} - 1} )}}$

It will now be appreciated that, as encoder emissivity goes up, itsreciprocal goes down, and heat transfer goes up; as rear coveremissivity goes up, its reciprocal goes down, and heat transfer goes up.

Testing has shown a substantial improvement in performance of the motorthrough such treatment of the internal components.

EXAMPLE 1

In testing, one motor experienced an increase in internal temperature of5.47 degrees C. as a result of the outside of the motor housing beingpainted white. As illustrated in the graph shown in FIG. 5, theperformance of the original motor with black outer is represented by theuppermost line. After painting the outside of the housing white, theperformance of the motor was degraded as shown by the lowermost line. Inbetween the uppermost line and the lowermost lines are two linesrepresenting improved performance based on treatment of the interiorsurface of the motor. In one test, the rear cover and the encoder wereboth anodized black. In another test, the rear cover was painted black,while the encoder was anodized black. Both configurations resulted inimproved motor performance as illustrated in the graph. Additionaltesting was performed on other motor models with similar results.

It will be appreciated that increasing the emissivity of the encoder andor housing results in better heat dissipation. Accordingly, aspects ofthe present disclosure can be applied to housings having exterior colorsother than white.

The exemplary embodiment has been described with reference to thepreferred embodiments. Obviously, modifications and alterations willoccur to others upon reading and understanding the preceding detaileddescription. It is intended that the exemplary embodiment be construedas including all such modifications and alterations insofar as they comewithin the scope of the appended claims or the equivalents thereof.

1. A motor assembly comprising: a housing including a motor portionhaving an opening for receiving a motor; a motor received in thehousing; an encoder coupled to the motor; an end cap for enclosing theopening of the motor portion of the housing, the end cap at leastpartially surrounding the encoder; wherein at least one of an interiorsurface of the end cap or an exterior surface of the encoder comprise amaterial having an emissivity greater than 0.9.
 2. The motor assembly ofclaim 1, wherein the end cap is aluminum, and the interior surface ofthe end cap is anodized black.
 3. The motor assembly of claim 1, whereinthe interior surface of the end cap is painted black.
 4. The motorassembly of claim 1, wherein both the encoder and the interior surfaceof the end cap are anodized black.
 5. The motor assembly of claim 1,wherein both the encoder and the interior surface of the end cap arepainted black.
 6. The motor assembly of claim 1, wherein the encoder ispainted black.
 7. The motor assembly of claim 1, wherein the motorassembly is a food-grade motor assembly having an exterior of thehousing painted white.
 8. An end cap for a motor assembly comprising: abody having an interior surface and an exterior surface; wherein theexterior surface is painted white; and wherein the interior surface hasan emissivity greater than 0.9.
 9. The end cap of claim 8, wherein theinterior surface is painted black.
 10. The end cap of claim 8, whereinthe body is aluminum, and wherein the interior surface is anodizedblack.
 11. The end cap of claim 8, wherein the body is aluminum, whereinthe interior and exterior surfaces of the body are anodized black, andwherein the exterior anodized black surface is painted white.
 12. Amethod of increasing heat transfer in a motor assembly comprising:providing a motor component; providing a motor housing for receiving themotor component; and increasing the emissivity of at least one of themotor component or an interior surface of the motor housing; wherein theemissivity is increased to at least 0.9.
 13. The method of claim 12,wherein the increasing the emissivity includes painting with blackpaint.
 14. The method of claim 12, wherein the increasing the emissivityincludes black anodizing.